My first realistic choice would be Earl Thomas. He's a ball hawking safety with good speed and is very solid against the run. He can add depth to a veteran safety tandem as he is groomed as a future starter and fill in at the nickel immediately. I really like this guy, really crossing my fingers he falls to us.
My second choice would be Dez Bryant if he falls to us. Dez would instantly make an impact in our offense in the slot position. Character issues are semi-concerning however the Bengals seem to deal well with that sort of thing (at least lately). I would expect him to have a Crabtree like season (though hopefully he'd actually play the first couple of games).
My third choice if neither of those two are available would be to trade down and pick up an extra second rounder. This a very deep draft at a lot of key positions for the Bengals and it saves a little cap room for next years free agency to resign players like JJo, etc. Being able to grab an extra player like Koa Misi, Eric Norwood, Rob Gronkowski, or Lamarr Houston would be awesome.
My 4th choice would be Sergio Kindle, he has both the great size and athleticism to project as the Bengals SAM LB in the 4-3 moving Rey Mauluga to the middle LB position. He is dangerous pass rusher and gets into the backfield at will. The defense could really benefit from Kindles ability to get to the QB, especially with some concerns regarding the ability to get pressure from the front 4.
If the Bengals can address needs at TE (either with Gronkowski or a trade for Scheffler with Denver), S, G, CB, WR, and OLB (or DE, basically someone who can get pressure on the QB) in the first four rounds while drafting a solid FB in the 5th I will consider the draft a tremendous success. With that said, I'm all for drafting the best player available within reason. After the first 5 or so picks the draft pretty much becomes a crapshoot...hopefully as the cards fall the Bengals can come out with a good hand just like they did last year.